Maximizing Power and Spectral Efficiencies for Layered and Conventional Modulations

ABSTRACT

Methods and apparatuses for maximizing power and spectral efficiencies in a wireless communication system are disclosed. The invention is particularly useful for layered modulation applications because power levels for such applications are relatively high. A layered modulation signal comprises an upper and a lower layer signal that interfere with each other within the same frequency band such that the upper layer signal can be demodulated directly from the layered modulation signal, and the lower layer signal can be demodulated after subtracting the first layer signal from the layered modulation signal. The invention applies one or more of the following four signal schemes in a communication signal including varying the symbol rate (rather than the code rate), reducing or eliminating the guard band, reducing excess signal bandwidth and employing layered modulation within the guard band of the legacy signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/207,462, filed Sep. 9, 2008, which is a continuation of U.S. patentapplication Ser. No. 10/532,619, filed Apr. 25, 2005, now issued Dec.30, 2008, as U.S. Pat. No. 7,471,735, by Ernest C. Chen, entitled“MAXIMIZING POWER AND SPECTRAL EFFICIENCIES FOR LAYERED AND CONVENTIONALMODULATIONS,” which application is a National Stage Application of andclaims the benefit under 35 U.S.C. §365 to PCT application US03/32800,filed Oct. 16, 2003, by Ernest C. Chen, entitled “MAXIMIZING POWER ANDSPECTRAL EFFICIENCIES FOR LAYERED AND CONVENTIONAL MODULATIONS,” whichclaims the benefit under 35 U.S.C. §119(e) of U.S. ProvisionalApplication Ser. No. 60/421,288, filed Oct. 25, 2002, by Ernest C. Chen,entitled “MAXIMIZING POWER AND SPECTRAL EFFICIENCIES FOR LAYERED ANDCONVENTIONAL MODULATIONS,” which applications are hereby incorporated byreference herein. U.S. patent application Ser. No. 10/532,619 is also acontinuation-in-part application of U.S. application Ser. No.09/844,401, filed Apr. 27, 2001, by Ernest C. Chen, entitled “LAYEREDMODULATION FOR DIGITAL SIGNALS,” now issued as U.S. Pat. No. 7,209,524,which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for power andspectral efficient transmission of signals, particularly signals usinglayered modulations.

2. Description of the Related Art

Digital signal communication systems have been used in various fields,including digital TV signal transmission, either terrestrial orsatellite. As the various digital signal communication systems andservices evolve, there is a burgeoning demand for increased datathroughput and added services. However, it is more difficult toimplement either improvement in old systems and new services when it isnecessary to replace existing legacy hardware, such as transmitters andreceivers. New systems and services are advantaged when they can utilizeexisting legacy hardware. In the realm of wireless communications, thisprinciple is further highlighted by the limited availability ofelectromagnetic spectrum. Thus, it is not possible (or at least notpractical) to merely transmit enhanced or additional data at a newfrequency.

The conventional method of increasing spectral capacity is to move to ahigher-order modulation, such as from quadrature phase shift keying(QPSK) to eight phase shift keying (8PSK) or sixteen quadratureamplitude modulation (16QAM). Unfortunately, QPSK receivers cannotdemodulate conventional 8PSK or 16QAM signals. As a result, legacycustomers with QPSK receivers must upgrade their receivers in order tocontinue to receive any signals transmitted with an 8PSK or 16QAMmodulation.

It is advantageous for systems and methods of transmitting signals toaccommodate enhanced and increased data throughput without requiringadditional frequency. In addition, it is advantageous for enhanced andincreased throughput signals for new receivers to be backwardscompatible with legacy receivers. There is further an advantage forsystems and methods which allow transmission signals to be upgraded froma source separate from the legacy transmitter.

It has been proposed that a layered modulation signal, transmittingnon-coherently both upper and lower layer signals, can be employed tomeet these needs. See Utility application Ser. No. 09/844,401. Suchlayered modulation systems allow higher information throughput withbackwards compatibility. However, even when backward compatibility isnot required (such as with an entirely new system), layered modulationcan still be advantageous because it requires a traveling wave tubeamplifier (TWTA) peak power significantly lower than that for aconventional 8PSK or 16QAM modulation format for a given throughput.

In the case of layered modulation systems designed to be backwardscompatible with legacy receivers and signals, such as existing satellitetelevision receivers for systems such as DIRECTV, the power requirementsto produce additional layered signals are excessive. Some systems andmethods have been recently proposed to facilitate layered modulationsignals. However, none of these systems propose systems or methods thataddress the high power requirements of implementing backwards compatiblelayered modulation signals, particularly with respect to satellitetelevision applications.

Accordingly, there is a need for systems and methods that mitigate thehigh power requirements of implementing backwards compatible layeredmodulation signals, particularly with respect to satellite televisionapplications. The present invention meets these and other needs asdescribed hereafter.

SUMMARY OF THE INVENTION

In the present invention four signal schemes are disclosed which greatlyalleviate the power requirements for layered modulation and provide foran increase in system information throughput, particularly in backwardscompatible layered modulation satellite television applications. In afirst signal scheme, the symbol rate (rather than code rate) is varied.In a second signal scheme, the guard band is reduced or eliminated. In athird signal scheme, the signal excess bandwidth ratio is reduced.Finally, in a fourth signal scheme, layered modulation is applied in theguard band. These distinct signal schemes can be used alone or incombination to improve the signal efficiency of in a satellitetelevision system thereby enabling a layered modulation transmission atconventional power levels.

In a typical method embodiment of the invention an upper layer signalwith a first excess bandwidth ratio is amplified to a first power levelwithin a frequency band. Next a lower layer signal with a second excessbandwidth ratio is amplified to a second power level within thefrequency band, the second power level being exceeded by the first powerlevel. Finally, a layered modulation signal is transmitted for at leastone receiver including the upper layer signal and the lower layersignal. The layered modulation signal comprises both the upper layersignal and the lower layer signal interfering with each other within thefrequency band such that the upper layer signal can be demodulateddirectly from the layered modulation signal and the lower layer signalcan be demodulated after subtracting the upper layer signal from thelayered modulation signal. Importantly, no guard band is used within thefrequency band.

In further embodiments of the invention the lower layer signal caninclude a lower layer code rate that is less than the upper layer coderate. In other further embodiments the excess bandwidth ratios of theupper and lower layer signals do not exceed 0.2. Typically, the upperlayer signal comprises a legacy signal in a satellite television systemwhich has a reduced excess bandwidth ratio over the original legacysignal.

In one exemplary embodiment the frequency band can include a thirdsignal having a third excess bandwidth ratio and occupying a majority ofthe frequency band. In this case, the third signal can be the legacysignal of a satellite television system. In one example, the upper andlower layer signals do not interfere with the third signal. In anothercase, the lower layer signal can interfere with the third signal as wellas the upper layer signal. In a further example, the bandwidth ratios ofthe upper and lower layer signal do not exceed 0.1 and the excessbandwidth ratio of the third signal does not exceed 0.2.

A typical system for transmitting the described layered signal includesa first amplifier amplifying the upper layer signal and a secondamplifier amplifying the lower layer signal. At least one antennatransmits the layered modulation signal comprising the upper and lowerlayer signals. The upper layer signal can be a legacy signal in asatellite television system. The first amplifier and the secondamplifier can operate in a common satellite or in different satellites.Similarly, the upper and lower layer signals can be transmitted from acommon antenna or different antennas.

A third amplifier can be added to amplify a third signal having a thirdexcess bandwidth ratio and occupying a majority of the frequency band.In this case, at least two of the three amplifiers can operate in acommon satellite. Also, two of the three signals of the layeredmodulation signal can be transmitted from a common antenna.

In one exemplary implementation, a three-phased throughput upgrade plancan be applied to an existing satellite television system incorporatingthe above signal schemes. The upgrade provides an information throughputincreases from approximately 50% to approximately 164% relative tolegacy systems, while maintaining receiver/decoder backwardcompatibility. The required TWTA power levels associated with the phasesare also progressive, from the current power level of approximately 240Watts to approximately 850 Watts. Thus, the same availability as that ofan existing satellite television system such as the DIRECTV continentalU.S. (CONUS) service can be attained.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a diagram illustrating an overview of a single satellite videodistribution system;

FIG. 2 is a block diagram showing a typical uplink configuration for asingle satellite transponder;

FIG. 3A is a diagram of a representative data stream;

FIG. 3B is a diagram of a representative data packet;

FIG. 4 is a block diagram showing one embodiment of the modulator forthe uplink signal;

FIG. 5 is a block diagram of an integrated receiver/decoder (IRD);

FIGS. 6A-6C are diagrams illustrating the basic relationship of signallayers in a layered modulation transmission;

FIGS. 7A-7C are diagrams illustrating a signal constellation of a secondtransmission layer over the first transmission layer after first layerdemodulation;

FIG. 8A is a diagram showing a system for transmitting and receivinglayered modulation signals;

FIG. 8B is a diagram showing an exemplary satellite transponder forreceiving and transmitting layered modulation signals;

FIG. 9 is a block diagram depicting one embodiment of an enhanced IRDcapable of receiving layered modulation signals;

FIG. 10A is a block diagram of one embodiment of the enhancedtuner/modulator and FEC encoder;

FIG. 10B depicts another embodiment of the enhanced tuner/modulatorwherein layer subtraction is performed on the received layered signal;

FIG. 11A depicts the relative power levels of example embodiments of thepresent invention;

FIG. 11B depicts the relative power level of an alternate embodiment;

FIG. 12 illustrates an exemplary computer system that could be used toimplement selected modules or functions the present invention;

FIG. 13 is a diagram illustrating exemplary method steps that can beused to practice one embodiment of the invention;

FIGS. 14A-14E illustrate the guard band as used in a layered modulationapplication;

FIGS. 15A and 15B illustrate the impact of excess bandwidth ration onsymbol timing error;

FIGS. 16A-16H illustrate some exemplary layered modulation schemes; and

FIGS. 17A-17C illustrate a three-phased implementation plan forupgrading an existing satellite television system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

1. Overview

As described in more detail hereafter, layered modulation (LM)reconstructs the upper layer signal and removes it from the receivedsignal to leave a lower-layer signal. Lower layer signal demodulationperformance requires good signal cancellation, which in turn requiresthe reconstructed signal to include accurate amplitude and phase effectsfrom signal propagation path, filter and low noise block (LNB). Valuesof these parameters change from receiver to receiver and therefore mustbe estimated at each receiver.

One difficulty with the implementation of the layered modulationtechniques, such as disclosed in Utility application Ser. No.09/844,401, filed Apr. 27, 2001, by Ernest C. Chen, entitled “LAYEREDMODULATION FOR DIGITAL SIGNALS,” is that the upper layer signal requiresexcessive satellite TWTA power beyond the current levels for a typicalcontinental United States (CONUS) coverage. The present inventionreduces the required powers to levels to current conventional TWTA powerlimits. Therefore, there is no need to wait for TWTA power technology tofurther develop before layered modulation can be implemented. Inaddition, the disclosed signal schemes further increase the informationthroughput with layered modulation since the entire bandwidth is usedmore efficiently.

The layered modulation technique as previously disclosed in Utilityapplication Ser. No. 09/844,401 established that the upper layer signalmust carry a power substantially higher than that of the lower layersignal in order for the technique to operate. Typically, suchbackwards-compatible (BWC) applications need more power than non-BWCapplications for the upper layer signal. Exemplary deployment scenariosrequire power levels of upper layer signal significantly beyondsatellite TWTA power technology for BWC applications.

As an example, the DIRECTV and GALAXY LATIN AMERICA systems combinedhave more than 10 million subscribers receiving QPSK signals fromsatellites. The conventional method of increasing information throughputover existing transponders would directly switching to a higher ordermodulation scheme such as 8PSK or 16 QAM. Unfortunately, this approachwould require a changeover of all IRDs in the field to be able toreceive the new signal. In contrast, deployment with the techniques ofthis invention (and Utility application Ser. No. 09/844,401), in thecontext of layered modulation, would allow existing IRDs to continuereceiving the legacy signal without modification. New and/or upgradedsubscribers would employ a new IRD to receive the new signal in additionto the legacy signal, both of which share the bandwidth. The potentialsavings of this transition is hundreds of million dollars, representingthe costs of mandatory replacement of all existing IRDs. In addition, achangeover of all IRDs over a short time period, as required by theconventional approach, would be logistically infeasible.

In addition, the layered modulation technique can be used incommunication systems outside of satellite television systems such asDIRECTV. For example, the very small aperture terminal (VSAT) throughputfor new customers could be increased in a BWC mode without interruptingthe service to the tens of thousands of existing customers. Anotherexample would be throughput increases for two-way voice and datacommunications systems using geosynchronous and low Earth orbit (LEO)satellites. Still other possibilities include BWC and non-BWCapplications of digital terrestrial broadcasting, digital cable, andcable modem services.

In situations where BWC is not required, layered modulation can also beused to provide higher throughputs than conventional waveforms using thesame power levels. Using QPSK and/or 8PSK for modulation layers for anew-service system, there will be no need for highly linear transponderTWTAs and/or special methods to adequately compensate for TWTAnonlinearity. Layered modulation can therefore achieve the high spectralefficiency of the 16 QAM modulation with its two-layered QPSKmodulation.

Various embodiments of this invention which effectively reduce powerrequirements for layered modulation make the layered modulationtechnique even more attractive in terms of power and bandwidthefficiency in many applications. In the sections hereafter, an exemplarysatellite video distribution system and associate hardware implementinglayered modulation are described. The system and hardware can employ thepower and spectral efficient signal schemes of the present invention.

2. Video Distribution System

FIG. 1 is a diagram illustrating an overview of a single satellite videodistribution system 100. The video distribution system 100 is comprisedof a control center 102 in communication with an uplink center 104 via aground or other link 114 and with a subscriber receiver station 110 viaa public switched telephone network (PSTN) or other link 120. Thecontrol center 102 provides program material (e.g. video programs, audioprograms and data) to the uplink center 104 and coordinates with thesubscriber receiver stations 110 to offer, for example, pay-per-view(PPV) program services, including billing and associated decryption ofvideo programs.

The uplink center 104 receives program material and program controlinformation from the control center 102, and using an uplink antenna 106and transmitter 105, transmits the program material and program controlinformation to the satellite 108 via uplink signal 116. The satellitereceives and processes this information, and transmits the videoprograms and control information to the subscriber receiver station 110via downlink signal 118 using transmitter 107. The subscriber receivingstation 110 receives this information using the outdoor unit (ODU) 112,which includes a subscriber antenna and a low noise block converter(LNB).

In one embodiment, the subscriber receiving station antenna is an18-inch slightly oval-shaped Ku-band antenna. The slight oval shape isdue to the 22.5 degree offset feed of the LNB (low noise blockconverter) which is used to receive signals reflected from thesubscriber antenna. The offset feed positions the LNB out of the way soit does not block any surface area of the antenna minimizing attenuationof the incoming microwave signal.

The video distribution system 100 can comprise a plurality of satellites108 in order to provide wider terrestrial coverage, to provideadditional channels, or to provide additional bandwidth per channel. Inone embodiment of the invention, each satellite is comprised of 16transponders to receive and transmit program material and other controldata from the uplink center 104 and provide it to the subscriberreceiving stations 110. Using data compression and multiplexingtechniques the channel capabilities, two satellites 108 working togethercan receive and broadcast over 150 conventional (non-HDTV) audio andvideo channels via 32 transponders.

While the invention disclosed herein will be described with reference toa satellite-based video distribution system 100, the present inventionmay also be practiced with terrestrial-based transmission of programinformation, whether by broadcasting means, cable, or other means.Further, the different functions collectively allocated among thecontrol center 102 and the uplink center 104 as described above can bereallocated as desired without departing from the intended scope of thepresent invention.

Although the foregoing has been described with respect to an embodimentin which the program material delivered to the subscriber 122 is video(and audio) program material such as a movie, the foregoing method canbe used to deliver program material comprised of purely audioinformation or other data as well.

2.1 Uplink Configuration

FIG. 2 is a block diagram showing a typical uplink configuration for asingle satellite 108 transponder, showing how video program material isuplinked to the satellite 108 by the control center 102 and the uplinkcenter 104. FIG. 2 shows three video channels (which may be augmentedrespectively with one or more audio channels for high fidelity music,soundtrack information, or a secondary audio program for transmittingforeign languages), a data channel from a program guide subsystem 206and computer data information from a computer data source 208.

Typical video channels are provided by a program source 200A-200C ofvideo material (collectively referred to hereinafter as programsource(s) 200). The data from each program source 200 is provided to anencoder 202A-202C (collectively referred to hereinafter as encoder(s)202). Each of the encoders accepts a program time stamp (PTS) from thecontroller 216. The PTS is a wrap-around binary time stamp that is usedto assure that the video information is properly synchronized with theaudio information after encoding and decoding. A PTS time stamp is sentwith each I-frame of the MPEG encoded data.

In one embodiment of the present invention, each encoder 202 is a secondgeneration Motion Picture Experts Group (MPEG-2) encoder, but otherdecoders implementing other coding techniques can be used as well. Thedata channel can be subjected to a similar compression scheme by anencoder (not shown), but such compression is usually either unnecessary,or performed by computer programs in the computer data source (forexample, photographic data is typically compressed into *.TIF files or*.JPG files before transmission). After encoding by the encoders 202,the signals are converted into data packets by a packetizer 204A-204F(collectively referred to hereinafter as packetizer(s) 204) associatedwith each program source 200.

The output data packets are assembled using a reference from the systemclock 214 (SCR), and from the conditional access manager 210, whichprovides the service channel identifier (SCID) to the packetizers 204for use in generating the data packets. These data packets are thenmultiplexed into serial data and transmitted.

2.2 Broadcast Data Stream Format and Protocol

FIG. 3A is a diagram of a representative data stream. The first packet302 is comprised of information from video channel 1 (data coming from,for example, the first video program source 200A). The next packet 304is comprised of computer data information that was obtained, for examplefrom the computer data source 208. The next packet 306 is comprised ofinformation from video channel 5 (from one of the video program sources200). The next packet 308 is comprised of program guide information suchas the information provided by the program guide subsystem 206. As shownin FIG. 3A, null packets 310 created by the null packet module 212 maybe inserted into the data stream as desired followed by further datapackets 312, 314, 316 from the program sources 200.

Referring back to FIG. 2, the data stream therefore is comprised of aseries of packets (302-316) from any one of the data sources (e.g.program sources 200, program guide subsystem 206, computer data source208) in an order determined by the controller 216. The data stream isencrypted by the encryption module 218, modulated by the modulator 220(typically using a QPSK modulation scheme), and provided to thetransmitter 105, which broadcasts the modulated data stream on afrequency bandwidth to the satellite via the antenna 106. The receiver500 at the receiver station 110 receives these signals, and using theSCID, reassembles the packets to regenerate the program material foreach of the channels.

FIG. 3B is a diagram of a data packet. Each data packet (e.g. 302-316)is 147 bytes long, and is comprised of a number of packet segments. Thefirst packet segment 320 is comprised of two bytes of informationcontaining the SCID and flags. The SCID is a unique 12-bit number thatuniquely identifies the data packet's data channel. The flags include 4bits that are used to control other features. The second packet segment322 is made up of a 4-bit packet type indicator and a 4-bit continuitycounter. The packet type generally identifies the packet as one of thefour data types (video, audio, data, or null). When combined with theSCID, the packet type determines how the data packet will be used. Thecontinuity counter increments once for each packet type and SCID. Thenext packet segment 324 is comprised of 127 bytes of payload data, whichin the cases of packets 302 or 306 is a portion of the video programprovided by the video program source 200. The final packet segment 326is data required to perform forward error correction.

FIG. 4 is a block diagram showing one embodiment of the modulator 220.The modulator 220 optionally is comprised of a forward error correction(FEC) encoder 404 which accepts the first signal symbols 402 and addsredundant information that are used to reduce transmission errors. Thecoded symbols 405 are modulated by modulator 406 according to a firstcarrier 408 to produce an upper layer modulated signal 410. Secondsymbols 420 are likewise provided to an optional second FEC encoder 422to produce coded second symbols 424. The coded second symbols 424 areprovided to a second modulator 414, which modulates the coded secondsignals 424 according to a second carrier 416 to produce a lower layermodulated signal 418. The upper layer modulated signal 410 and the lowerlayer modulated signal 418 are therefore uncorrelated. Thus, the upperlayer signal 410 and the lower layer signal 418 can be transmitted toseparate transponders on one or more satellites 108 via separate uplinksignals 116. Thus, the lower layer signal 418 can be implemented from aseparate satellite 108 that receives a separate uplink signal 116.However, in the downlink signal 118 the upper layer signal 410, must bea sufficiently greater amplitude signal than the lower layer signal 418,to maintain the signal constellations shown in FIG. 6 and FIG. 7.

It should be noted that it may be more efficient to retrofit an existingsystem by using a transponder on a separate satellite 108 to transmitthe lower layer downlink signal over the existing legacy downlink signalrather than replacing the legacy satellite with one that will transmitboth downlink signal layers. Emphasis can be given to accommodating thedownlink legacy signal in implementing a layered downlink broadcast.

2.3 Integrated Receiver/Decoder

FIG. 5 is a block diagram of an integrated receiver/decoder (IRD) 500(also hereinafter alternatively referred to as receiver 500). Thereceiver 500 is comprised of a tuner/demodulator 504 communicativelycoupled to an ODU 112 having one or more low noise blocks (LNBs) 502.The LNB 502 converts the 12.2- to 12.7 GHz downlink 118 signal from thesatellites 108 to, e.g., a 950-1450 MHz signal required by the IRD's 500tuner/demodulator 504. Typically, the LNB 502 may provide either a dualor a single output. The single-output LNB 502 has only one RF connector,while the dual output LNB 502 has two RF output connectors and can beused to feed a second tuner 504, a second receiver 500, or some otherform of distribution system.

The tuner/demodulator 504 isolates a single, digitally modulated 24 MHztransponder signal, and converts the modulated data to a digital datastream. The digital data stream is then supplied to a forward errorcorrection (FEC) decoder 506. This allows the IRD 500 to reassemble thedata transmitted by the uplink center 104 (which applied the forwarderror correction to the desired signal before transmission to thesubscriber receiving station 110) verifying that the correct data signalwas received, and correcting errors, if any. The error-corrected datamay be fed from the FEC decoder module 506 to the transport module 508via an 8-bit parallel interface.

The transport module 508 performs many of the data processing functionsperformed by the IRD 500. The transport module 508 processes datareceived from the FEC decoder module 506 and provides the processed datato the video MPEG decoder 514 and the audio MPEG decoder 517. As neededthe transport module employs system RAM 528 to process the data. In oneembodiment of the present invention, the transport module 508, videoMPEG decoder 514 and audio MPEG decoder 517 are all implemented onintegrated circuits. This design promotes both space and powerefficiency, and increases the security of the functions performed withinthe transport module 508. The transport module 508 also provides apassage for communications between the microcontroller 510 and the videoand audio MPEG decoders 514, 517. As set forth more fully hereinafter,the transport module also works with the conditional access module (CAM)512 to determine whether the receiver 500 is permitted to access certainprogram material. Data from the transport module 508 can also besupplied to external communication module 526.

The CAM 512 functions in association with other elements to decode anencrypted signal from the transport module 508. The CAM 512 may also beused for tracking and billing these services. In one embodiment of thepresent invention, the CAM 512 is a removable smart card, havingcontacts cooperatively interacting with contacts in the IRD 500 to passinformation. In order to implement the processing performed in the CAM512, the IRD 500, and specifically the transport module 508 provides aclock signal to the CAM 512.

Video data is processed by the MPEG video decoder 514. Using the videorandom access memory (RAM) 536, the MPEG video decoder 514 decodes thecompressed video data and sends it to an encoder or video processor 516,which converts the digital video information received from the videoMPEG module 514 into an output signal usable by a display or otheroutput device. By way of example, processor 516 may comprise a NationalTV Standards Committee (NTSC) or Advanced Television Systems Committee(ATSC) encoder. In one embodiment of the invention both S-Video andordinary video (NTSC or ATSC) signals are provided. Other outputs mayalso be utilized, and are advantageous if high definition programming isprocessed.

Audio data is likewise decoded by the MPEG audio decoder 517. Thedecoded audio data may then be sent to a digital to analog (D/A)converter 518. In one embodiment of the present invention, the D/Aconverter 518 is a dual D/A converter, one for the right and leftchannels. If desired, additional channels can be added for use insurround sound processing or secondary audio programs (SAPs). In oneembodiment of the invention, the dual D/A converter 518 itself separatesthe left and right channel information, as well as any additionalchannel information. Other audio formats may similarly be supported. Forexample, other audio formats such as multi-channel DOLBY DIGITAL AC-3may be supported.

A description of the processes performed in the encoding and decoding ofvideo streams, particularly with respect to MPEG and JPEGencoding/decoding, can be found in Chapter 8 of “Digital TelevisionFundamentals,” by Michael Robin and Michel Poulin, McGraw-Hill, 1998,which is hereby incorporated by reference herein.

The microcontroller 510 receives and processes command signals from aremote control, an IRD 500 keyboard interface, and/or other suitableinput device 524. The microcontroller 510 receives commands forperforming its operations from a processor programming memory, whichpermanently stores such instructions for performing such commands. Theprocessor programming memory may comprise a read only memory (ROM) 538,an electrically erasable programmable read only memory (EEPROM) 522 or,similar memory device. The microcontroller 510 also controls the otherdigital devices of the IRD 500 via address and data lines (denoted “A”and “D” respectively, in FIG. 5).

The modem 540 connects to the customer's phone line via the PSTN port120. It calls, e.g. the program provider, and transmits the customer'spurchase information for billing purposes, and/or other information. Themodem 540 is controlled by the microprocessor 510. The modem 540 canoutput data to other I/O port types including standard parallel andserial computer I/O ports.

The present invention also is comprised of a local storage unit such asthe video storage device 532 for storing video and/or audio dataobtained from the transport module 508. Video storage device 532 can bea hard disk drive, a read/writable compact disc of DVD, a solid stateRAM, or any other suitable storage medium. In one embodiment of thepresent invention, the video storage device 532 is a hard disk drivewith specialized parallel read/write capability so that data may be readfrom the video storage device 532 and written to the device 532 at thesame time. To accomplish this feat, additional buffer memory accessibleby the video storage 532 or its controller may be used. Optionally, avideo storage processor 530 can be used to manage the storage andretrieval of the video data from the video storage device 532. The videostorage processor 530 may also comprise memory for buffering datapassing into and out of the video storage device 532. Alternatively orin combination with the foregoing, a plurality of video storage devices532 can be used. Also alternatively or in combination with theforegoing, the microcontroller 510 can also perform the operationsrequired to store and or retrieve video and other data in the videostorage device 532.

The video processing module 516 input can be directly supplied as avideo output to a viewing device such as a video or computer monitor. Inaddition, the video and/or audio outputs can be supplied to an RFmodulator 534 to produce an RF output and/or 8 vestigal side band (VSB)suitable as an input signal to a conventional television tuner. Thisallows the receiver 500 to operate with televisions without a videooutput.

Each of the satellites 108 is comprised of a transponder, which acceptsprogram information from the uplink center 104, and relays thisinformation to the subscriber receiving station 110. Known multiplexingtechniques are used so that multiple channels can be provided to theuser. These multiplexing techniques include, by way of example, variousstatistical or other time domain multiplexing techniques andpolarization multiplexing. In one embodiment of the invention, a singletransponder operating at a single frequency band carries a plurality ofchannels identified by respective service channel identification (SCID).

Preferably, the IRD 500 also receives and stores a program guide in amemory available to the microcontroller 510. Typically, the programguide is received in one or more data packets in the data stream fromthe satellite 108. The program guide can be accessed and searched by theexecution of suitable operation steps implemented by the microcontroller510 and stored in the processor ROM 538. The program guide may includedata to map viewer channel numbers to satellite transponders and servicechannel identifications (SCIDs), and also provide TV program listinginformation to the subscriber 122 identifying program events.

The functionality implemented in the IRD 500 depicted in FIG. 5 can beimplemented by one or more hardware modules, one or more softwaremodules defining instructions performed by a processor, or a combinationof both.

The present invention provides for the modulation of signals atdifferent power levels and advantageously for the signals to benon-coherent from each layer. In addition, independent modulation andcoding of the signals may be performed. Backwards compatibility withlegacy receivers, such as a quadrature phase shift keying (QPSK)receiver is enabled and new services are provided to new receivers. Atypical new receiver of the present invention uses two demodulators andone remodulator as will be described in detail hereafter.

In a typical backwards-compatible embodiment of the present invention,the legacy QPSK signal is boosted in power to a higher transmission (andreception) level. This creates a power “room” in which a new lower layersignal may operate. The legacy receiver will not be able to distinguishthe new lower layer signal, from additive white Gaussian noise, and thusoperates in the usual manner. The optimum selection of the layer powerlevels is based on accommodating the legacy equipment, as well as thedesired new throughput and services.

The new lower layer signal is provided with a sufficient carrier tothermal noise ratio to function properly. The new lower layer signal andthe boosted legacy signal are non-coherent with respect to each other.Therefore, the new lower layer signal can be implemented from adifferent TWTA and even from a different satellite. The new lower layersignal format is also independent of the legacy format, e.g., it may beQPSK or 8PSK, using the conventional concatenated FEC code or using anew Turbo code. The lower layer signal may even be an analog signal.

The combined layered signal is demodulated and decoded by firstdemodulating the upper layer to remove the upper carrier. The stabilizedlayered signal may then have the upper layer FEC decoded and the outputupper layer symbols communicated to the upper layer transport. The upperlayer symbols are also employed in a remodulator, to generate anidealized upper layer signal. The idealized upper layer signal is thensubtracted from the stable layered signal to reveal the lower layersignal. The lower layer signal is then demodulated and FEC decoded andcommunicated to the lower layer transport.

Signals, systems and methods using the present invention may be used tosupplement a pre-existing transmission compatible with legacy receivinghardware in a backwards-compatible application or as part of apreplanned layered modulation architecture providing one or moreadditional layers at a present or at a later date.

2.4 Layered Signals

FIGS. 6A-6C illustrate the basic relationship of signal layers in areceived layered modulation transmission. FIG. 6A illustrates an upperlayer signal constellation 600 of a transmission signal showing thesignal points or symbols 602. FIG. 6B illustrates the lower layer signalconstellation of symbols 604 over the upper layer signal constellation600 where the layers are coherent (or synchronized). FIG. 6C illustratesa lower layer signal 606 of a second transmission layer over the upperlayer constellation where the layers are non-coherent. The lower layer606 rotates about the upper layer constellation 602 due to the relativemodulating frequencies of the two layers in a non-coherent transmission.Both the upper and lower layers rotate about the origin due to the firstlayer modulation frequency as described by path 608.

FIGS. 7A-7C are diagrams illustrating a non-coherent relationshipbetween a lower transmission layer over the upper transmission layerafter upper layer demodulation. FIG. 7A shows the constellation 700before the first carrier recovery loop (CRL) of the upper layer and Theconstellation rings 702 rotate around the large radius circle indicatedby the dashed line. FIG. 7B shows the constellation 704 after CRL of theupper layer where the rotation of the constellation rings 702 isstopped. The constellation rings 702 are the signal points of the lowerlayer around the nodes 602 of the upper layer. FIG. 7C depicts a phasedistribution of the received signal with respect to nodes 602.

Relative modulating frequencies of the non-coherent upper and lowerlayer signals cause the lower layer constellation to rotate around thenodes 602 of the upper layer constellation to form rings 702. After thelower layer CRL this rotation is eliminated and the nodes of the lowerlayer are revealed (as shown in FIG. 6B). The radius of the lower layerconstellation rings 702 is indicative of the lower layer power level.The thickness of the rings 702 is indicative of the carrier to noiseratio (CNR) of the lower layer. As the two layers are non-coherent, thelower layer may be used to transmit distinct digital or analog signals.

FIG. 8A is a diagram showing a system for transmitting and receivinglayered modulation signals. Separate transmitters 107A, 107B (whichinclude TWTAs to amplify the signals), as may be located on any suitableplatform, such as satellites 108A, 108B, are used to non-coherentlytransmit different layers of a signal of the present invention. Eachsatellite includes additional transmitters 107C, 107D which can be usedto transmit additional signals (from additional received uplink signals)to be used in the frequency bandwidth of the layered signal as detailedhereafter. Uplink signals 116 are typically transmitted to eachsatellite 108A, 108B from one or more uplink centers 104 with one ormore transmitters 105 via an antenna 106.

FIG. 8B is a diagram illustrating an exemplary satellite transponder 107for receiving and transmitting layered modulation signals on a satellite108. The uplink signal 116 is received by the satellite 108 and passedthrough a input multiplexer (IMUX) 814. Following this the signal isamplified with a travelling wave tube amplifier (TWTA) 816 and thenthrough an output muliplexer (OMUX) 818 before the downlink signal 118is transmitted to the receivers 802, 500.

The layered signals 808A, 808B (e.g. multiple downlink signals 118) arereceived at receiver antennas 812A, 812B, such as satellite dishes, eachwith a low noise block (LNB) 810A, 810B where they are then coupled tointegrated receiver/decoders (IRDs) 500, 802. For example, firstsatellite 108A and transmitter 107A can transmit an upper layer legacysignal 808A and second satellite 108B and transmitter 107B can transmita lower layer signal 808B. Although both signals 808A, 808B arrive ateach antenna 812A, 812B and LNB 810A, 810B, only the layer modulationIRD 802 is capable of decoding both signals 808A, 808B. The legacyreceiver 500 is only capable of decoding the upper layer legacy signal808A; the lower layer signal 808B appears only as noise to the legacyreceiver 500.

Because the signal layers can be transmitted non-coherently, separatetransmission layers may be added at any time using different satellites108A, 108B or other suitable platforms, such as ground-based or highaltitude platforms. Thus, any composite signal, including new additionalsignal layers will be backwards compatible with legacy receivers 500,which will disregard the new signal layers. To ensure that the signalsdo not interfere, the combined signal and noise level for the lowerlayer must be at or below the allowed noise floor for the upper layer atthe particular receiver antenna 812A, 812B.

Layered modulation applications include backwards compatible andnon-backwards compatible applications. “Backwards compatible” in thissense, describes systems in which legacy receivers 500 are not renderedobsolete by the additional signal layer(s). Instead, even if the legacyreceivers 500 are incapable of decoding the additional signal layer(s),they are capable of receiving the layered modulated signal and decodingthe original signal layer. In these applications, the pre-existingsystem architecture is accommodated by the architecture of theadditional signal layers. “Non-backwards compatible” describes a systemarchitecture which makes use of layered modulation, but the modulationscheme employed is such that pre-existing equipment is incapable ofreceiving and decoding the information on additional signal layer(s).

The pre-existing legacy IRDs 500 decode and make use of data only fromthe layer (or layers) they were designed to receive, unaffected by theadditional layers. However, as will be described hereafter, the legacysignals may be modified to optimally implement the new layers. Thepresent invention may be applied to existing direct satellite serviceswhich are broadcast to individual users in order to enable additionalfeatures and services with new receivers without adversely affectinglegacy receivers and without requiring additional signal frequency.

2.5 Demodulator and Decoder

FIG. 9 is a block diagram depicting one embodiment of an enhanced IRD802 capable of receiving layered modulation signals. The IRD includesmany similar components as that of the legacy IRD 500 of FIG. 5.However, the enhanced IRD 802 includes a feedback path 902 in which theFEC decoded symbols are fed back to a enhanced modifiedtuner/demodulator 904 and transport module 908 for decoding both signallayers as detailed hereafter.

FIG. 10A is a block diagram of one embodiment of the enhancedtuner/modulator 904 and FEC encoder 506. FIG. 10A depicts receptionwhere layer subtraction is performed on a signal where the upper layercarrier has already been demodulated. The upper layer of the receivedcombined signal 1016 from the LNB 502, which may contain legacymodulation format, is provided to and processed by an upper layerdemodulator 1004 to produce the stable demodulated signal 1020. Thedemodulated signal 420 is communicatively coupled to a FEC decoder 402which decodes the upper layer to produce the upper layer symbols whichare output to an upper layer transport module 908. The upper layersymbols are also used to generate an idealized upper layer signal. Theupper layer symbols may be produced from the decoder 402 after Viterbidecode (BER<10⁻³ or so) or after Reed-Solomon (RS) decode (BER<10⁻⁹ orso), in typical decoding operations known to those skilled in the art.The upper layer symbols are provided via feedback path 902 from theupper layer decoder 402 to a remodulator 406 which effectively producesan idealized upper layer signal. The idealized upper level signal issubtracted from the demodulated upper layer signal 1020.

In order for the subtraction to yield a clean small lower layer signal,the upper layer signal must be precisely reproduced. The modulatedsignal may have been distorted, for example, by traveling wave tubeamplifier (TWTA) non-linearity or other non-linear or linear distortionsin the transmission channel. The distortion effects are estimated fromthe received signal after the fact or from TWTA characteristics whichmay be downloaded into the IRD in AM-AM and/or AM-PM maps 1014, used toeliminate the distortion.

A subtractor 1012 then subtracts the idealized upper layer signal fromthe stable demodulated signal 1020. This leaves the lower-power secondlayer signal. The subtractor 1012 may include a buffer or delay functionto retain the stable demodulated signal 1020 while the idealized upperlayer signal is being constructed. The second layer signal isdemodulated by the lower level demodulator 1010 and FEC decoded bydecoder 1008 according to its signal format to produce the lower layersymbols, which are provided to the transport module 908.

FIG. 10B depicts another embodiment wherein layer subtraction isperformed on the received layered signal (prior to upper layerdemodulation). In this case, the upper layer demodulator 1004 producesthe upper carrier signal 1022 (as well as the stable demodulated signaloutput 1020). An upper carrier signal 1022 is provided to theremodulator 1006. The remodulator 1006 provides the remodulated signalto the non-linear distortion mapper 1018 which effectively produces anidealized upper layer signal. Unlike the embodiment shown in FIG. 10A,in this embodiment the idealized upper layer signal includes the upperlayer carrier for subtraction from the received combined signal 808A,808B.

Other equivalent methods of layer subtraction will occur to thoseskilled in the art and the present invention should not be limited tothe examples provided here. Furthermore, those skilled in the art willunderstand that the present invention is not limited to two layers;additional layers may be included. Idealized upper layers are producedthrough remodulation from their respective layer symbols and subtracted.Subtraction may be performed on either the received combined signal or ademodulated signal. Finally, it is not necessary for all signal layersto be digital transmissions; the lowest layer may be an analogtransmission.

The following analysis describes the exemplary two layer demodulationand decoding. It will be apparent to those skilled in the art thatadditional layers may be demodulated and decoded in a similar manner.The incoming combined signal is represented as:

${s_{UL}(t)} = {{f_{U}\left( {M_{U}{\exp \left( {{j\; \omega_{U}t} + \theta_{U}} \right)}{\sum\limits_{m = {- \infty}}^{\infty}\; {S_{Um}{p\left( {t - {mT}} \right)}}}} \right)} + {f_{L}\left( {M_{L}{\exp \left( {{{j\omega}_{L}t} + \theta_{L}} \right)}{\sum\limits_{m = {- \infty}}^{\infty}\; {S_{Lm}{p\left( {t - {mT} + {\Delta \; T_{m}}} \right)}}}} \right)} + {n(t)}}$

where, M_(U) is the magnitude of the upper layer QPSK signal and M_(L)is the magnitude of the lower layer QPSK signal and M_(L)<<M_(U). Thesignal frequencies and phase for the upper and lower layer signals arerespectively ω_(U),θ_(U) and ω_(U),θ_(U). The symbol timing misalignmentbetween the upper and lower layers is ΔT_(m). p(t−mT) represents thetime shifted version of the pulse shaping filter p(t) 414 employed insignal modulation. QPSK symbols S_(Um) and S_(Lm) are elements of

$\left\{ {{\exp \left( {j\frac{n\; \pi}{2}} \right)},{n = 0},1,2,3} \right\}.$

f_(U)(•) and f_(L)(•) denote the distortion function of the TWTAs forthe respective signals.

Ignoring f_(U)(•) and f_(L)(•) and noise n(t), the following representsthe output of the demodulator 1004 to the FEC decoder 1002 afterremoving the upper carrier:

${s_{UL}^{\prime}(t)} = {{M_{U}{\sum\limits_{m = {- \infty}}^{\infty}\; {S_{Um}{p\left( {t - {mT}} \right)}}}} + {M_{L}\exp \left\{ {{{j\left( {\omega_{L} - \omega_{U}} \right)}t} + \theta_{L} - \theta_{U}} \right\} {\sum\limits_{m = {- \infty}}^{\infty}\; {S_{Lm}{p\left( {t - {mT} + {\Delta \; T_{m}}} \right)}}}}}$

Because of the magnitude difference between M_(U) and M_(L), the upperlayer decoder 402 disregards the M_(L) component of the s′_(UL)(t).

After subtracting the upper layer from s_(UL)(t) in the subtractor 1012,the following remains:

${s_{L}(t)} = {M_{L}\exp \left\{ {{{j\left( {\omega_{L} - \omega_{U}} \right)}t} + \theta_{L} - \theta_{U}} \right\} {\sum\limits_{m = {- \infty}}^{\infty}\; {S_{Lm}{p\left( {t - {mT} + {\Delta \; T_{m}}} \right)}}}}$

Any distortion effects, such as TWTA nonlinearity effects are estimatedfor signal subtraction. In a typical embodiment of the presentinvention, the upper and lower layer frequencies are substantiallyequal. Significant improvements in system efficiency can be obtained byusing a frequency offset between layers.

Using the present invention, two-layered backward compatible modulationwith QPSK doubles a current 6/7 rate capacity by adding a TWTAapproximately 6.2 dB above an existing TWTA power. New QPSK signals maybe transmitted from a separate transmitter, from a different satellitefor example. In addition, there is no need for linear travelling wavetube amplifiers (TWTAs) as with 16QAM. Also, no phase error penalty isimposed on higher order modulations such as 8PSK and 16QAM.

3.0 Power Levels of Modulation Layers

In a layered modulation system, the relationship between the individualmodulation layers can be structured to facilitate backward compatibleapplications. Alternately, a new layer structure can be designed tooptimize the combined efficiency and/or performance of the layeredmodulation system.

3.1 Backward Compatible Applications

FIG. 11A depicts the relative power levels 1100 of example embodimentsof the present invention. FIG. 11A is not a scale drawing. Thisembodiment doubles the pre-existing rate 6/7 capacity by using a TWTA6.2 dB above a pre-existing TWTA equivalent isotropic radiated power(EIRP) and second TWTA 2 dB below the pre-existing TWTA power. Thisembodiment uses upper and lower QPSK layers which are non-coherent. Acode rate of 6/7 is also used for both layers. In this embodiment, thesignal of the legacy QPSK signal 1102 is used to generate the upperlayer 1104 and a new QPSK layer is the lower layer 1110. The CNR of thelegacy QPSK signal 1102 is approximately 7 dB. In the present invention,the legacy QPSK signal 1102 is boosted in power by approximately 6.2 dBbringing the new power level to approximately 13.2 dB as the upper layer1104. The noise floor 1106 of the upper layer is approximately 6.2 dB.The new lower QPSK layer 1110 has a CNR of approximately 5 dB. The totalsignal and noise of the lower layer is kept at or below the tolerablenoise floor 1106 of the upper layer. The power boosted upper layer 1104of the present invention is also very robust, making it resistant torain fade. It should be noted that the invention may be extended tomultiple layers with mixed modulations, coding and code rates.

In an alternate embodiment of this backwards compatible application, acode rate of 2/3 may be used for both the upper and lower layers 1104,1110. In this case, the CNR of the legacy QPSK signal 1102 (with a coderate of 2/3) is approximately 5.8 dB. The legacy signal 1102 is boostedby approximately 5.3 dB to approximately 11.1 dB (4.1 dB above thelegacy QPSK signal 1102 with a code rate of 2/3) to form the upper QPSKlayer 1104. The new lower QPSK layer 1110 has a CNR of approximately 3.8dB. The total signal and noise of the lower layer 1110 is kept at orbelow approximately 5.3 dB, the tolerable noise floor 1106 of the upperQPSK layer. In this case, overall capacity is improved by 1.55 and theeffective rate for legacy IRDs will be 7/9 of that before implementingthe layered modulation.

In a further embodiment of a backwards compatible application of thepresent invention the code rates between the upper and lower layers1104, 1110 may be mixed. For example, the legacy QPSK signal 502 may beboosted by approximately 5.3 dB to approximately 12.3 dB with the coderate unchanged at 6/7 to create the upper QPSK layer 1104. The new lowerQPSK layer 1110 may use a code rate of 2/3 with a CNR of approximately3.8 dB. In this case, the total capacity relative to the legacy signal1102 is approximately 1.78. In addition, the legacy IRDs will suffer nosignificant rate decrease.

3.2 Non-Backward Compatible Applications

As previously discussed the present invention may also be used in“non-backward compatible” applications. In a first example embodiment,two QPSK layers 1104, 1110 are used each at a code rate of 2/3. Theupper QPSK layer 504 has a CNR of approximately 4.1 dB above its noisefloor 1106 and the lower QPSK layer 1110 also has a CNR of approximately4.1 dB. The total code and noise level of the lower QPSK layer 1110 isapproximately 5.5 dB. The total CNR for the upper QPSK signal 1104 isapproximately 9.4 dB, merely 2.4 dB above the legacy QPSK signal rate6/7. The capacity is approximately 1.74 compared to the legacy rate 6/7.

FIG. 11B depicts the relative power levels of an alternate embodimentwherein both the upper and lower layers 1104, 1110 are below the legacysignal level 1102. The two QPSK layers 1104, 1110 use a code rate of1/2. In this example, the upper QPSK layer 1104 is approximately 2.0 dBabove its noise floor 1106 of approximately 4.1 dB. The lower QPSK layerhas a CNR of approximately 2.0 dB and a total code and noise level at orbelow 4.1 dB. The capacity of this embodiment is approximately 1.31compared to the legacy rate 6/7.

4. Hardware Environment

FIG. 12 illustrates an exemplary computer system 1200 that could be usedto implement selected modules and/or functions of the present invention.The computer 1202 is comprised of a processor 1204 and a memory 1206,such as random access memory (RAM). The computer 1202 is operativelycoupled to a display 1222, which presents images such as windows to theuser on a graphical user interface 1218B. The computer 1202 may becoupled to other devices, such as a keyboard 1214, a mouse device 1216,a printer, etc. Of course, those skilled in the art will recognize thatany combination of the above components, or any number of differentcomponents, peripherals, and other devices, may be used with thecomputer 1202.

Generally, the computer 1202 operates under control of an operatingsystem 1208 stored in the memory 1206, and interfaces with the user toaccept inputs and commands and to present results through a graphicaluser interface (GUI) module 1218A. Although the GUI module 1218A isdepicted as a separate module, the instructions performing the GUIfunctions can be resident or distributed in the operating system 1208,the computer program 1210, or implemented with special purpose memoryand processors. The computer 1202 also implements a compiler 1212 whichallows an application program 1210 written in a programming languagesuch as COBOL, C++, FORTRAN, or other language to be translated intoprocessor 1204 readable code. After completion, the application 1210accesses and manipulates data stored in the memory 1206 of the computer1202 using the relationships and logic that was generated using thecompiler 1212. The computer 1202 also optionally is comprised of anexternal communication device such as a modem, satellite link, Ethernetcard, or other device for communicating with other computers.

In one embodiment, instructions implementing the operating system 1208,the computer program 1210, and the compiler 1212 are tangibly embodiedin a computer-readable medium, e.g., data storage device 1220, whichcould include one or more fixed or removable data storage devices, suchas a zip drive, floppy disc drive 1224, hard drive, CD-ROM drive, tapedrive, etc. Further, the operating system 1208 and the computer program1210 are comprised of instructions which, when read and executed by thecomputer 1202, causes the computer 1202 to perform the steps necessaryto implement and/or use the present invention. Computer program 1210and/or operating instructions may also be tangibly embodied in memory1206 and/or data communications devices 1230, thereby making a computerprogram product or article of manufacture according to the invention. Assuch, the terms “article of manufacture,” “program storage device” and“computer program product” as used herein are intended to encompass acomputer program accessible from any computer readable device or media.

Those skilled in the art will recognize many modifications may be madeto this configuration without departing from the scope of the presentinvention. For example, those skilled in the art will recognize that anycombination of the above components, or any number of differentcomponents, peripherals, and other devices, may be used with the presentinvention.

5. Modulation Schemes for Maximizing Power and Spectral Efficiency

The layered modulation (LM) technique described above typically requiresthe use of transmitters of transponders in satellites 108A, 108B, withthe upper layer transponder having greater power output than thoseassociated with ordinary modulation techniques. Typically, the uppersignal layer 808A must be modulated by a carrier of substantially higherpower than the lower signal layer 808B. Also, backwards compatible (BWC)applications typically require more power than non-BWC applications forthe upper signal layer 808A.

Embodiments of the present invention utilize one or more of four definedsignal schemes to improve the power and spectral efficiency of system.Such improvements allow for layered modulation systems to operate atconventional TWTA power levels. The four signal schemes are described indetail below. The signal schemes can be employed separately or incombination to achieve improved performance. In the first signal scheme,the symbol rate of the lower layer signal is reduced below the symbolrate of the upper layer signal (or vice versa if non-backwardscompatible). In the second signal scheme, the guard band providingagainst adjacent channel interference is reduced or eliminated. In thethird signal scheme, the excess signal bandwidth ratio, α, is reduced.In the last signal scheme, multiple signal layers can be used to providea new data stream in the guard band of the legacy signal.

FIG. 13 is a diagram illustrating an exemplary method 1300 that can beused to practice one embodiment of the invention. At step 1302, an upperlayer signal is amplified with a first excess bandwidth ratio at a firstpower level within a frequency band. At step 1304, a lower layer signalis amplified with a second excess bandwidth ratio at a second powerlevel within the frequency band, the second power level being exceededby the first power level. Finally, at step 1306, a layered modulationsignal is transmitted for at least one receiver, the layered modulationsignal comprising both the upper layer signal and the lower layer signalinterfering with each other within the frequency band such that theupper layer signal can be demodulated directly from the layeredmodulation signal and the lower layer signal can be demodulated aftersubtracting the upper layer signal from the layered modulation signal,wherein substantially no guard band is used within the frequency band.The foregoing method can be applied to implement the signal schemesdescribed hereafter in the system architecture detailed above.

Typically, the upper layer signal comprises a legacy signal in asatellite television system that has a reduced excess bandwidth ratioover the original legacy signal. For example, the original legacy signalmay have an excess bandwidth ratio of 0.2. Accordingly, the new layeredsignals will each have an independent bandwidth ratio that does notexceed 0.2. The excess bandwidth ratio for any of the layered signalscan be further reduced not to exceed 0.1. In addition, the upper andlower layer signals can be amplified and transmitted from a commonsatellite and/or antenna or from different satellites and/or antennas.

5.1 Symbol Rate Variation

The first modulation scheme involves reducing the symbol rate of thelower layer signal, e.g., the new lower layer signal 808B operating overthe upper layer legacy signal 808A. The symbol rate in a digital signalrelates to the signal power concentrated over smaller bandwidth;doubling the symbol rate doubles the throughput and carrier power(requiring that adequate available bandwidth). By reducing the symbolrate below that of the upper layer signal 808A, the lower layer signal808B occupies a narrower bandwidth. This means that a smaller amount ofinterference will be exhibited at the legacy signal. Thus, the legacysignal 808A can be operated at a lower power level than would otherwisebe required to be received by a legacy receiver 500. However, a reducedsymbol rate will also reduce the throughput for the lower layer signal808B.

In contrast, merely reducing the code rate of the lower layer signal808B does not reduce signal bandwidth. The spectrum of a digital signalrelates to the signal power spread across the signal bandwidthregardless of the code rate. A code rate reduction would reduce therequired CNR. In fact, continued reduction of the code rate wouldeventually drive the carrier-to-noise ratios (CNR) below an acceptablethreshold required to ensure carrier lock for signal demodulation Inaddition, doubling the carrier power only increases Shannon capacity byapproximately 1 bps/Hz at high CNR; the required power increases morethan linearly with throughput. It is for these reasons that changing thesymbol rate can be more attractive than changing the code rate in manycases.

5.2 Guard Band Reduction

A second modulation scheme requires reducing or eliminating the guardband. In the current DIRECTV broadcast satellite frequency plan, a guardband of 5.16 MHz exists between adjacent transponders of the samepolarization. This is a legacy configuration from earlier satellitecommunication standards for analog FM transmission. The FM communicationstandard requires a relatively high CNR (on the order of 14 dB), and istherefore more susceptible to spectral re-growth effects from satelliteTWTA non-linearity and other imperfections. In fact, the designed guardband has provided more than enough margin for the FM signal to rejectspread signal energy due to adjacent channel interference (ACI).

In comparison, the current digital Direct Broadcast Systems (DBS) signalrequires a CNR on the order of 7.6 dB with prevailing QPSK modulationand concatenated forward error correction (FEC) codes. With the adventof turbo-like codes, such as turbo product codes, serial/parallelconcatenated convolutional codes and low-density parity check codes,which provide higher coding gains than the conventional codes, therequired CNR drops even lower (to around 5.4 dB for the same modulationand a similar code rate). In the following, turbo-like codes arereferred to as advanced FEC codes in contrast with conventional codes.Again, other factors being equal, signals with smaller CNRs are lesssusceptible to noise and interference effects. For example, a computersimulation on out-of-band (OOB) signal power ratios of a typical TWTAnonlinearity yields an OOB ratio of approximately −20 dB at ±12 MHz withan α of 0.2, and an OOB ratio of approximately −20 dB at ±11 MHz with anα of 0.1. Both these simulations assume linearized TWTAs and areconservative, based upon a “brick wall” filter for the undesired signal.

FIGS. 14A-14E illustrate spectral outgrowth effects into the guard band.FIG. 14A is an exemplary computer simulated signal spectrum without TWTAnonlinearity and with an α of 0.2. FIG. 14B illustrates the amplitudeand phase characteristics of an exemplary “linearized” satellite TWTA.FIG. 14C illustrates exemplary signal spectrum after processing throughan output multiplexer (OMUX) on a satellite with a of 0.1. Thus, thesignal has been processed through an input multiplexer (IMUX), thelinearized TWTA and the OMUX. FIG. 14D illustrates the out-of-bandsignal power ratio versus the cutoff frequency with an α of 0.2.Spectral re-growth is mostly due to TWTA non-linearity. With a 5.16 MHzguard band, the OOB ratio is approximately −23 dB at the maximumfrequency f_(max) at ±17.2 MHz. Without the guard band, the OOB ratio isapproximately −20 dB at the f_(max) of ±12 MHz. FIG. 14E illustrates theout-of-band signal power ratio versus the cutoff frequency with an α of0.1. Here, with the guard band, f_(max) is ±18.2 MHz and the OOB ratiois approximately −24 dB. Without the guard band, f_(max) is ±11 MHz andthe OOB ratio is approximately −20 dB (little changed from the α=0.2case).

Accordingly, the existing guard band for DIRECTV (and other DBS systems)may be reduced or even eliminated with only a small impact oncommunication performance. For example, eliminating the DIRECTV guardband could increase spectral efficiency by a factor of approximately 22%(from the ratio of 29.16/24). The throughput increase is achieved byincreasing the symbol rate with this ratio without the need to increasethe code rate which would require more power.

5.3 Excess Signal Bandwidth Ratio Reduction

Excess bandwidth reduces inter-symbol interference (ISI) that comes fromsymbol timing recovery and other errors from the demodulator. ISI is aform of “self-interference;” degradation on CNR increases with the CNRvalue. An excess bandwidth ratio of 0.2 is used in current DIRECTVsystems. For similar reasons discussed above as applied to guard bandreduction, degradation from ISI on CNR is not as severe for lower CNRs.Analysis and simulation show that the α for digital satellitecommunication may go as low as 0.1 without significant performancedegradation. For reference, the advanced television systems committee(ATSC) terrestrial digital TV standards need a much higher CNR (thusmore susceptible to ISI effects), yet the standard only stipulates anexcess bandwidth ratio of about 0.1.

Reducing the excess bandwidth ratio from 0.2 to 0.1 for DIRECTV easilyincreases spectral efficiency by as much as 9% (from the ratio of1.2/1.1). Consistent with the guard band reduction scheme, throughputincrease from a reduced excess bandwidth ratio is achieved by increasingthe symbol rate with the above ratio. The combined throughput increasefrom a guard band reduction and an excess bandwidth ratio reduction isapproximately 32%. Although a lower CNR from using an advanced FEC codewill result in greater timing recovery errors, computer simulations showthat current excess bandwidth provides an adequate margin. A slightlyincreased impact on CNR is observed with a pessimistic root mean square(RMS) timing error of approximately 0.075 times the symbol interval(TWTA non-linearity not included). Thus, an excess bandwidth ratio of0.1 reduces the signal CNR by approximately 0.151 dB. In comparison, anexcess bandwidth ratio of 0.2 reduces the signal CNR by approximately0.148 dB, and an excess bandwidth ratio of 0.35 reduces the signal CNRby approximately 0.136 dB. TWTA non-linearity flattens the signalwaveform and therefore increases tracked timing errors. This can becompensated by imposing slightly higher linearity requirements on newTWTAs. The smaller lower layer signal power required allows the TWTA tooperate closer to its linear region.

FIGS. 15A and 15B illustrate the impact of excess bandwidth ratio onsymbol timing error by computer simulations. FIG. 15A illustrates thesymbol timing error with an excess bandwidth ratio of 0.1 for a rootraised cosine filter. The ISI is calculated by sampling from adjacentsymbols off zero-crossing points. The CNR reduction is calculated bysampling the signal off-peak. FIG. 15B illustrates the symbol timingerror with an excess bandwidth ratio of 0.2. The results are verysimilar to that of FIG. 15A.

5.4 Layered Modulation in the Guard Band

If two new layered signals are added substantially in the guard band ofthe legacy upper layer signal, spectral efficiency can be doubled with asmall increase in the noise floor from that of the legacy signal. Thismeans it is possible to add additional throughput with backwardcompatibility and with a small increase in legacy signal power. Therewill be some mutual impact between signals in guard and legacy bands dueto spectral re-growth, particularly when the guard band and/or theexcess bandwidth ratio are reduced from their original values assuggested by this invention. The worst case scenario with respect toimpact is to the new lower layer signal in the guard band from thelegacy signal (e.g. desensitization of approximately 0.9 dB), aconsequence of the significantly higher power of the legacy signal. Onesolution to mitigate this effect is to increase the power levels of thelayered signals to overcome the degradation from legacy signal. Theimpact on the legacy signal from the new lower layer signal is lesssevere (e.g. receiver desensitization of approximately 0.2 dB). Both newsignal layers can include an advanced FEC code. In addition,implementation of this modulation scheme will provide the infrastructurethat can later be used to convert the system to a non-backwardscompatible modulation scheme with maximized spectral efficiency.

5.5 Exemplary Applications of Modulation Schemes

When the modulation schemes described in sections 5.1 through 5.4 arecombined, spectral efficiency of the legacy system can be increased byas much as 50% while remaining backwards compatible with the legacysystem. When the above modulation schemes are selectively applied tonon-BWC applications, better power and spectral efficiencies alsofollow. As an example, use of layered modulation can increase spectralefficiency by as much as 184% with only approximately 4.3 dB of increaserelative to legacy signal power. In comparison, the 8-PSK system (withan advanced FEC code) would only achieve approximately 72% increase inspectral efficiency, while requiring a 1.2 dB power increase.

FIGS. 16A-16H illustrate some exemplary layered modulationimplementations. FIG. 16A illustrates a basic layered modulationimplementation using a single carrier frequency for both layers with anexcess bandwidth ratio of 0.2 for both the upper and lower layersignals. The code rate is 6/7 for both signals and spectral efficiencyis 200% relative to the a legacy signal. Both the upper layer signal1600 and the lower layer signal 1602 occupy the same frequency band1604. In this case, a guard band 1606A, 1606B is indicated by theabsence of signal on both sides of the frequency band 1604.

FIG. 16B illustrates spectral efficiency of modulation schemes of thepresent invention compared with the basic layered modulationimplementation. The horizontal axis is spectral efficiency relative tothat of the legacy signal, and the vertical axis is the effective noisefloor as seen by the upper layer signal (the lower the noise floor, theless power the upper layer signal requires). In this case, the carrierlock requirement is ignored. Both the upper and lower layer signals havean excess bandwidth ratio of 0.2. Spectral efficiency improves to amaximum of 222% of the legacy throughput as the lower layer signalincludes the guard band when compared with the basic layered modulationimplementation. The plot shows a curve for varying the symbol rate aswell as varying the code rate. Varying the code rate is shown to be morepower efficient than varying the symbol rate for the upper layer signal.

FIG. 16C illustrates spectral efficiency of modulation schemes of thepresent invention compared with the basic layered modulationimplementation, but with the carrier lock requirement considered. Here,also, both the upper and lower layer signals have an excess bandwidthratio of 0.2. Again, spectral efficiency improves to a maximum of 222%as the lower layer signal includes the guard band when compared with thebasic layered modulation implementation. The upper layer carrier signaloperates at approximately 1080 W with a symbol rate of 20 MHz. The lowerlayer carrier signal operates at approximately 176 W with a symbol rateof 24.3 MHz. Similar to FIG. 16B, the plot shows a curve for varying thesymbol rate as well as varying the code rate. Varying the code rate ismore power efficient than varying the symbol rate down to approximately50%, when the carrier becomes a problem. The maximum upper layer powerincrease is approximately 6.5 dB.

Because reducing the code rate and symbol rate results in the leastspectral efficiencies, a layered modulation implementation can beginwith both the code rate and symbol rate maximized. In this case, a CNRof at least 6.5 dB should be used for the upper layer signal. Next,spectral efficiency can be reduced to a desired or affordable level interms of power requirements. As the symbol rate is reduced, the totalnoise introduced by the lower layer decreases linearly. The lower layersignal frequency should be positioned to minimize spectral overlap withthe upper layer signal frequency. As the code rate is reduced, the totalnoise introduced by the lower layer signal decreases at a greater thanlinear rate. Also, reducing the code rate is limited by the required CNRfloor for the carrier lock of the lower layer signal, e.g.,approximately 1.2 dB for QPSK without pilots. Crossover in spectralefficiency generally exists between the methods of reducing the coderate and reducing the symbol rate. Accordingly, the code rate can beselected above crossover spectral efficiency and the symbol rateselected below crossover spectral efficiency.

FIG. 16D illustrates an exemplary layered modulation signal scheme wherethe lower layer signal is disposed in the guard band of the upper layersignal. The lower layer signal begins at the edge of the frequency bandto minimize interference into the upper layer signal. In this example,the lower layer signal (guard band signal) has an excess bandwidth ratioof 0.2 and a power level of approximately 72 W. The upper layer signal(legacy signal) has an excess bandwidth ratio of 0.2 and a power levelof 398 W. The code rate is 6/7 for the legacy upper layer signal and thenew-service lower layer signal. This example yields a spectralefficiency of approximately 150% compared with a legacy signal. As isshown, substantially no guard band is used within the frequency band1604.

FIG. 16E illustrates spectral efficiency of the exemplary layeredmodulation signal scheme of FIG. 16D. No interference is shown with thelower layer signal bandwidth up to 5.2 MHz. The code rate is fixed at6/7 for the lower layer signal. Spectral efficiency improves to about150% over a legacy signal with a +2.2 dB over the legacy signal power.However, spectral efficiency of 222% is obtained with a +6.5 dB powerlevel over the legacy signal power. Spectral re-growth is ignored here.Note that varying the symbol rate is more power efficient than varyingthe code rate up to a 180% spectral efficiency factor. The maximum powerincrease of the upper layer signal is approximately 6.5 dB.

FIG. 16F illustrates another exemplary layered modulation signal schemewhere the lower layer signal is disposed in the guard band of the upperlayer signal. In this case the lower layer signal has an excessbandwidth ratio of 0.1 and a power level of approximately 72 W. Herealso the lower layer signal begins at the edge of the frequency band tominimize interference into the upper layer signal. The upper layersignal (legacy signal) has an excess bandwidth ratio of 0.2 and a powerlevel of 380 W. The code rate is 6/7. Again, as shown, substantially noguard band is used within the frequency band 1604.

FIG. 16G illustrates spectral efficiency of the exemplary layeredmodulation signal scheme of FIG. 16F. Less interference into the upperlayer signal by the lower layer signal is shown, as compared with FIG.16E. In this example, spectral efficiency improves about 150% with a+2.0 dB over the legacy signal power. Spectral efficiency of 232% isobtained with a +6.5 dB power level over the legacy signal power. Theincrease in spectral efficiency is because a higher symbol rate isavailable than with the previous example (although the lower layersignal requires approximately +0.4 dB). Spectral re-growth is againignored here. Note that varying the symbol rate is more power efficientthan varying the code rate for up to a 185% spectral efficiency. Themaximum power increase of the upper layer signal is approximately 6.5dB.

FIG. 16H illustrates spectral efficiency of the exemplary layeredmodulation signal scheme where a two-layered signal is applied in theguard band. Here a steep curve is exhibited with varying symbol rate.The signal scheme is most efficient at lower symbol rates and there islittle spectral overlap with the legacy signal. Spectral efficiency isabout 150% with no power increase required over the legacy signal power.Spectral efficiency of 172% is obtained with only a +2 dB power levelover the legacy signal power.

From the foregoing, it is seen that power can be optimized in a layeredmodulation system by varying the symbol and/or code rate for backwardscompatible applications. The appropriate change in the symbol and/orcode rates depends upon the designed spectral efficiency improvement andwhether the new signal is backwards compatible. Lower spectralefficiency improvement (e.g. up to +80%) should employ two-layermodulation of the lower layer signal (e.g. in the guard band). Moderatespectral efficiency improvement should employ a single lower layersignal with a varied symbol rate. High spectral efficiency improvementshould employ a single lower layer signal with a varied code rate.

FIGS. 17A-17C illustrate an exemplary three-phased implementation planfor upgrading an existing satellite television broadcast system. FIG.17A illustrates a first phase where a two-layered signal is added to theguard band. Three transponders are required, one for the legacy signaland two new transponders for each layer of the guard band signal. Thelayered guard band signals each may have an excess bandwidth ratio of0.1, while the legacy signal retains an excess bandwidth ratio of 0.2.The CNR for the upper and lower layer signals of the guard band are 11.9dB and 5.4 dB respectively. The legacy signal employs a CNR of 7.6 dB.Accordingly, the transponder for the legacy signal need not be upgradedto implement the first phase although the layered signals may beoperated from an interim satellite. Spectral efficiency improves to 150%of the legacy. In this case, the upper layer signal 1600 and the lowerlayer signal 1602 share the frequency band 1604 with a third signal1608, the legacy signal, which occupies a majority of the frequencyband. In this phase, the upper layer signal 1600 and the lower layersignal 1602 are distinct from the third signal, i.e., the layeredsignals do not interfere with the third signal 1608. Effectively, theupper layer signal and the lower layer signal are employed within theguard band of the old legacy signal. Thus, in the new signal,substantially all guard band is used within the frequency band 1604.

FIG. 17B illustrates a second phase where two upper layer signals arespanned by one lower layer signal. One of the upper layer signals is thelegacy signal which must have a raised power level to overcome the“noise” of the lower layer signal. Accordingly, the transponder for thelegacy signal will need to be upgraded, e.g., migrated to a newsatellite. In addition, the lower layer signal may also be migrated toanother transponder, e.g., on the same new satellite. The CNR for theupper and lower layer signals of the guard band are 11.9 dB and 5.4 dBrespectively, while the legacy signal employs a CNR of 13.1 dB. At thisphase spectral efficiency increases to 230%. As with the first phase,the upper layer signal 1600 and the lower layer signal 1602 share thefrequency band 1604 with a third signal 1608, the legacy signal, whichdominates the frequency band. Here, the lower layer signal 1602interferes with the legacy signal as well as the upper layer signal1600. Again, substantially all guard band is used within the frequencyband 1604.

FIG. 17C illustrates a third phase where layered modulation of the wholespectrum is implemented with new signals. The upper layer signalsubstantially takes the place of the legacy signal while the lower layersignal substantially takes the place of the signal previously layeredwith the upper layer guard band signal. The CNR for the new upper andlower layer signals are 11.9 dB and 5.4 dB respectively. The excessbandwidth ratio is 0.1 for both signals. At this final phase, spectralefficiency increases to 264%. Here the third signal of the second phasebecomes the upper layer signal 1600 over the lower layer signal 1602.Substantially all guard band is used within the frequency band 1604.

This concludes the description including the preferred embodiments ofthe present invention. The foregoing description of the preferredembodiment of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Many modificationsand variations are possible in light of the above teaching.

It is intended that the scope of the invention be limited not by thisdetailed description, but rather by the claims appended hereto. Theabove specification, examples and data provide a complete description ofthe manufacture and use of the apparatus and method of the invention.Since many embodiments of the invention can be made without departingfrom the scope of the invention, the invention resides in the claimshereinafter appended.

1. A method for transmitting signals and receiving signals in a legacyreceiver and a layered modulation receiver, comprising: amplifying anupper layer signal portion of a downlink signal at a first power levelwithin a frequency band of the downlink signal; amplifying a lower layersignal portion of the downlink signal at a second power level within thefrequency band of the downlink signal, the second power level beingexceeded by the first power level; transmitting a layered modulationsignal, the layered modulation signal comprising both the amplifiedupper layer signal portion and the amplified lower layer signal portioninterfering with each other within the frequency band, wherein theamplified upper layer signal portion is transmitted by a firsttransponder and the amplified lower layer signal portion is transmittedby a second transponder and transmitting at the same polarization;demodulating the upper layer signal directly from the layered modulationsignal in a legacy receiver and in a layered modulation receiver; anddemodulating the lower layer signal after subtracting a remodulatedupper layer signal from the layered modulation signal in the layeredmodulation receiver and not in the legacy receiver; wherein the lowerlayer signal portion includes a lower layer symbol rate, the upper layersignal portion includes an upper layer symbol rate, and the lower layersymbol rate is lower than the upper layer symbol rate.
 2. The method ofclaim 1, wherein the upper layer signal and the lower layer signal arenon-coherent.
 3. The method of claim 1, wherein substantially no guardband is used within the frequency band.
 4. The method of claim 1,wherein the upper layer signal portion of the downlink signal has afirst excess bandwidth ratio and the lower layer signal portion of thedownlink signal has a second excess bandwidth ratio, and wherein atleast one of the first excess bandwidth ratio and the second excessbandwidth ratio does not exceed 0.1.
 5. The method of claim 1, wherein:the upper layer signal portion of the downlink signal has a first excessbandwidth ratio and the lower layer signal portion of the downlinksignal has a second excess bandwidth ratio; the layered modulationsignal is transmitted by at least one antenna; the method furthercomprises the step of amplifying a third signal portion of the downlinksignal, the third signal portion having a third excess bandwidth ratioand being transmitted by the at least one antenna to occupy a majorityof the frequency band of the downlink signal; and the upper layer signaland the lower layer signal together comprise a non-legacy signal,transmitted in a remainder of the frequency band of the downlink signal;the third signal portion comprises a legacy signal; the third excessbandwidth ratio does not exceed 0.2; and the first excess bandwidthratio and the second bandwidth ratio do not exceed 0.1.
 6. The method ofclaim 1, wherein the upper layer signal comprises a legacy signal in asatellite television system, and the lower layer signal comprises anon-legacy signal in the satellite television system.
 7. The method ofclaim 1, wherein the upper layer signal portion is modulated with afirst modulation scheme and the lower layer signal portion is modulatedwith a second modulation scheme different to the first modulationscheme.
 8. A satellite television system for transmitting signals andreceiving signals, comprising: a first amplifier for amplifying an upperlayer signal portion of a downlink signal at a first power level withina frequency band of the downlink signal; a second amplifier foramplifying a lower layer signal portion of a downlink signal at a secondpower level within the frequency band of the downlink signal, the secondpower level being exceeded by the first power level; at least oneantenna for transmitting a layered modulation signal, the layeredmodulation signal comprising both the upper layer signal portion and thelower layer signal portion interfering with each other within thefrequency band, wherein the amplified upper layer signal portion istransmitted by a first transponder and the amplified lower layer signalis transmitted by a second transponder transmitting at the samepolarization as the first transponder; and a legacy receiver configuredto demodulate the upper layer signal portion from the layered modulationsignal and not configured to demodulate the lower layer signal portion;and a non-legacy receiver configured to demodulate the upper layersignal portion directly from the layered modulation signal andconfigured to demodulate the lower layer signal by subtracting aremodulated upper layer signal from the layered modulation signal;wherein the lower layer signal portion includes lower layer symbol rate,the upper layer signal includes an upper layer symbol rate and the lowerlayer symbol rate is less than the upper layer symbol rate.
 9. Thesystem of claim 8, wherein substantially no guard band is used withinthe frequency band.
 10. The system of claim 8, wherein the upper layersignal and the lower layer signal are non-coherent.
 11. The system ofclaim 8, wherein the upper layer signal comprises a legacy signal in thesatellite television system and the lower layer signal comprises anon-legacy signal in the satellite television system.
 12. The system ofclaim 8, wherein the upper layer signal portion is modulated with afirst modulation scheme and the lower layer signal portion is modulatedwith a second modulation scheme different to the first modulationscheme.
 13. The system of claim 8, wherein the upper layer portion ofthe downlink signal has a first excess bandwidth ratio and the lowerlayer signal portion of the downlink signal has a second excessbandwidth ratio, and wherein at least one of the first excess bandwidthratio and the second excess bandwidth ratio does not exceed 0.1
 14. Thesystem of claim 13, further comprising: means for amplifying a thirdsignal portion of the downlink signal, the third signal portion having athird excess bandwidth ratio and being transmitted by the at least oneantenna to occupy a majority of the frequency band of the downlinksignal; wherein the upper layer signal and the lower layer signaltogether comprise a non-legacy signal, transmitted in a remainder of thefrequency band of the downlink signal, and the third signal portioncomprises a legacy signal.
 15. The system of claim 14, wherein the andthe third excess bandwidth ratio does not exceed 0.2 and the firstexcess bandwidth ratio and the second bandwidth ratio do not exceed 0.1.16. The system of claim 8, wherein the second transponder is adjacentthe first transponder.